1. Field of the Invention
The present invention relates to an exhaust gas purification system of an internal combustion engine having a particulate filter in an exhaust passage. Specifically, the present invention relates to an exhaust gas purification system capable of efficiently regenerating a particulate filter by increasing temperature of the particulate filter.
2. Description of Related Art
In recent years, an exhaust gas purification system for inhibiting discharge of toxic components by treating exhaust gas discharged from an internal combustion engine by using a catalyst or a filter has been emphasized as one of measures for protecting the environment. For instance, an exhaust gas purification system, which has a diesel particulate filter (a DPF, hereafter) in an exhaust pipe and collects particulate matters discharged from a diesel engine with the DPF, is known. The DPF is regenerated by regularly combusting and eliminating the particulate matters accumulated in the DPF. Thus, the DPF can be used continuously.
The regeneration of the DPF is performed by increasing temperature of the DPF above a certain temperature (for instance, 600° C.) at which the particulate matters can be combusted, when the quantity of the particulate matters accumulated in the DPF (a particulate matter accumulation quantity, hereafter) reaches a predetermined value. The particulate matter accumulation quantity is calculated based on a pressure difference across the DPF, for instance. At that time, temperature increasing means performs post-injection, retardation of fuel injection timing, restriction of intake air or the like. The post-injection is an additional injection of a small amount of the fuel performed after a main injection, which is performed to operate the engine. However, these temperature increasing methods can deteriorate fuel consumption.
If the temperature of the DPF (DPF temperature T, hereafter) is low during the regeneration of the DPF, combustion speed of the particulate matters will be decreased as shown by a solid line “a” in FIG. 21. Accordingly, the regeneration of the DPF takes a long time and an amount of the fuel consumption is increased as shown by a broken line “b” in FIG. 21. If the DPF temperature T is high during the regeneration of the DPF, the combustion speed of the particulate matters is increased as shown by the solid line “a” in FIG. 21. In this case, the regeneration of the DPF is finished in a short time and the deterioration of the fuel consumption due to the regeneration of the DPF can be reduced as shown by the broken line “b” in FIG. 21. However, if the DPF temperature T is increased excessively, the DPF will be damaged or an oxidation catalyst supported on the DPF will be degraded because of the excessive temperature increase. An area “Ad” in FIG. 21 shows an area where the DPF can be damaged or the oxidation catalyst supported on the DPF can be degraded. Therefore, in order to inhibit the deterioration of the fuel consumption and to regenerate the DPF safely, the DPF temperature T has to be maintained in an appropriate range. Therefore, normally, temperature of the exhaust gas upstream or downstream of the DPF is sensed and the temperature increasing means is operated so that the sensed temperature approaches target temperature.
In a conventional technology disclosed in Unexamined Japanese Patent Application Publication No. H11-101122, an oxidation catalyst (a diesel oxidation catalyst: a DOC, hereafter) is serially disposed upstream of the DPF, and temperature of the exhaust gas upstream of the DPF and downstream of the DOC is sensed as the DPF temperature. If the DPF temperature (the temperature of the exhaust gas upstream of the DPF) shown by a thin line “b” in FIG. 22 exceeds a predetermined value (for instance, 500° C.) at a time point “tA”, temperature increasing operation is stopped. Then, if the DPF temperature shown by the thin line “b” becomes lower than the predetermined temperature (for instance, 500° C.) at a time point “tB”, the temperature increasing operation is started again. An “ON” state of a line “T-UP” in FIG. 22 represents a state in which the temperature increasing operation is performed by the temperature increasing means, and an “OFF” state of the line T-UP represents a state in which the temperature increasing operation is not performed.
However, the above technology performs an operation for merely switching the temperature increasing means, which performs the post-injection or the like, between the operated state and the stopped state. In this case, there is a possibility that the post-injection deteriorates the fuel consumption but the DPF is not substantially regenerated if the post-injection is performed but the DPF temperature decreases to low temperature (450° C. or under, for instance) because of a disturbance such as a change in an operating state of the engine. It is because the combustion speed of the particulate matters accumulated in the DPF is slow when the DPF temperature is low. In the above technology, it takes a long time to increase the DPF temperature to the proximity of the target temperature again in the case where the DPF temperature deviates from the target temperature largely during the regeneration of the DPF. As a result, the fuel consumption is deteriorated.
The DPF temperature is determined by a balance between heat generation through an oxidizing reaction of hydrocarbon due to a function of the oxidation catalyst, and heat radiation into the exhaust gas or a surrounding area. Therefore, in the case where a large amount of the exhaust gas passes through the DPF or in the case where the hydrocarbon is not supplied during deceleration and the like, the quantity of the radiated heat will exceed the quantity of the heat generated through the oxidizing reaction of the hydrocarbon, and the DPF temperature will decrease. If the DPF temperature once decreases, it takes a long time to achieve the target temperature TT even if the temperature increasing operation is started again.
If the temperature increasing operation is stopped at the time point tA in FIG. 22, low-temperature exhaust gas flows into the DOC and the heat generation through the oxidizing reaction of the hydrocarbon is stopped. Accordingly, the temperature of the DOC disposed upstream of the DPF decreases rapidly as shown by a broken line “a” in FIG. 22. The DPF has a greater heat capacity than the DOC. Therefore, the change in the actual temperature of the DPF shown by a heavy line “c” in FIG. 22 is delayed compared to the change in the DOC temperature shown by the broken line “a” in FIG. 22. Therefore, the sensed temperature does not decrease quickly as shown by the thin line “b”, and the DOC temperature further decreases as shown by the broken line “a” before the sensed temperature shown by the thin line “b” decreases. Even if the temperature increasing operation is resumed because of the decrease in the sensed temperature shown by the thin line “b” at the time point tB in FIG. 22, the low-temperature exhaust gas passing through the DOC flows into the DPF. Therefore, the decrease in the actual temperature of the DPF shown by the heavy line “c” does not stop immediately. The actual temperature of the DPF starts increasing after the DOC temperature shown by the broken line “a” becomes high as a result of the resumption of the temperature increasing operation.
If a state in which the DPF temperature is lower than a predetermined temperature (for instance, 450° C. or under) continues during the regeneration of the DPF, the regeneration of the DPF will be extended and the fuel consumption will be deteriorated. In order to avoid such a situation, the DPF temperature should be preferably increased quickly to the target temperature TT (500° C., for instance) and maintained near the target temperature TT in the regeneration of the DPF.